1. Field of the Invention
This invention relates generally to methods for treating contaminated crystalline surfaces to render them atomically clean. More particularly, it relates to a method for removing surface contaminants, such as oxygen and carbon, from silicon and germanium in order to achieve clean, well-ordered surfaces.
2. Problem
In the preparation of crystalline silicon and germanium articles for research and industrial applications, it is important from the standpoint of electrical properties that the surfaces be both clean and well-ordered. That is, it is important that the surface be as free as possible from impurities and that the near-surface layer have a structure approximating its as-grown structure. Various techniques have been employed for the cleaning of crystalline silicon and germanium surfaces but none of them has been as rapid and effective as desired. The conventional cleaning techniques for such surfaces include the following: in-vacuum sputtering followed by thermal annealing (probably the most extensively used technique at present), in-vacuum cleavage, in-vacuum evaporation, chemical etching, and electron scrubbing. [R. W. Roberts. British Journal of Applied Physics 14, 537 (1963)] Although sputtering, as with an ion beam, removes impurities from such surfaces, it must be followed by an annealing operation to convert the resulting amorphous surface to a well-ordered structure. Usually, the sputtering-and-annealing cycle is repeated several times, and the total cleaning time for a typical off-the-shelf wafer is a matter of hours.
The use of a pulsed laser beam to remove sulfur and oxygen from nickel-crystal surfaces is reported in the following reference: Journal of Applied Physics 40, No. 12, pp. 4776-4781 (1969). The degree of removal of the impurities was determined by comparing before-and-after low-energy electron diffraction patterns (LEED patterns) for the nickel surface. However, no spectroscopic techniques capable of identifying the atomic species or the quantities of such species present in the surface region were used. The same reference also describes an experiment in which a surface of a silicon crystal was irradiated with laser pulses. In that experiment, however, the crystal was throughly cleaned before irradiation. That is, prior to irradiation the crystal was (a) mechanically polished, (b) chemically polished, and (c) annealed at 1000.degree.C. for 6 hours (to remove absorbed gases). LEED patterns of the resulting cleaned surface were obtained before and after irradiation. The reference does not assert or imply that cleaning of the silicon was effected by the irradiation. In fact, it is well known in the surface-physics art that--with respect to silicon, at least--LEED patterns do not reflect the presence or absence of particular atomic species.
The use of pulsed laser beams to process silicon crystals is described in various patents and the references cited therein, including the following: U.S. Pat. No. 4,147,563, "Method for Forming P-N Junctions and Solar Cells by Laser-Beam Processing," Apr. 3, 1979; U.S. Pat. No. 4,181,538 (Ser. No. 945,925), "Method for Making Defect-Free Zone by Laser-Annealing of Doped Silicon," filed on Sept. 26, 1978; U.S. patent application Ser. No. 80,725, "Method of Producing Ohmic Contacts on Semiconducting Oxides," filed on Sept. 25, 1979; and U.S. patent application Ser. No. 96,871, "Method Utilizing Laser-Processing for the Growth of Epitaxial P-N Junctions," filed on Nov. 23, 1979.
3. Objects of the Invention
Accordingly, it is an object of this invention to provide a novel method for removing surface impurities from crystalline germanium and silicon surfaces.
It is another object to provide a novel method for preparing clean crystalline silicon and germanium to be used in semiconductor applications.
It is another object to provide a novel method for converting crystalline silicon and germanium surfaces contaminated with macro amounts of oxygen and/or carbon to surfaces which are atomically clean with respect to those contaminants.
It is another object to provide a rapid and reliable method for efficiently removing impurities--e.g., oxygen and carbon--from a surface of a crystalline silicon or germanium article.
Other objects, advantages, and novel features will become apparent from the description and the figures.